good-bye

early 12c., "a fortunate day," also, generally, "good fortune;" from good (adj.) + day (n.). As a salutation in parting, haue godne day "have good day" is recorded from c. 1200; good day as a greeting is from late 14c.

late 14c., adewe, from Old French a Dieu, a Deu, shortened from phrases such as a dieu (vous) commant "I commend (you) to God," from a "to" (see ad-) + dieu "God," from Latin deum, accusative of deus "god" (from PIE *deiwos "god" (from root *dyeu- "to shine").

Originally it was said to the party left (farewell was to the party setting forth), but in English it came to be used as a general parting salutation. As a noun, "expression of kind wishes upon departure," late 14c. Compare the native parting salutation good-bye, a contraction of God be with ye.

1709, shortened form of good-bye. Also compare lullaby. Reduplication bye-bye is recorded from 1736; as a sound used to lull a child to sleep it is attested from c. 1500.